Lactide is the raw material for the production of biodegradable plastics. High purity lactide is subjected to a ring opening polymerization to form polylactide (PLA). The main use is currently in medical applications, where it applied in three different solid forms, (i) as bone plates and screws for the fixation of fractures or similar purposes, (ii) as solid with tensile strength to produce sutures (stitching material) and (iii) the glue form being used for joining membranes or thins skins in humans. Due to its high strength against UV radiation, it is also used in fibers, as packaging material of e.g. foods and as a solvent. From an environmental protection perspective there is a growing interest for a broader use of PLA in packaging applications since it readily degrades into environmentally acceptable by-products.
Many impurities and specifically hydroxylic impurities like e.g. acids or water have a large chain-stopping potential during polymerization and can thus prevent the attainment of the desired high molecular weights of the PLA. Since the tolerable levels of all impurities in the lactide for the synthesis of PLA are remarkably low, it is therefore economically and ecologically attractive to provide a method that can achieve such required purities in a single and efficient operation.
In addition, the combination of a certain amount of impurities along with high temperature exposure, favors an unwanted early start of the polymerization of the lactide, and results in a decrease of lactide yield and an increase of the product acidity. This is the reason why melt crystallization process are typically preferred against other process operating at higher temperatures: product losses are significant in case of purification by distillation. While the lower operation temperature of the melt crystallization process are preferred as mentioned above, the frequent repetition of multiple batch wise layer melt crystallization steps with the respective melting operations at elevated temperatures can also worsen the lactide yield
Lactide exist as an L-isomer, and D-isomer and an M-isomer. While it is typically preferred to use a polymer with a high amount of L-isomer in the production of PLA, a certain racemic mixture of specific isomers would provide different properties of the final polymer; for certain specific applications it may thus be commercially interesting to produce a defined mixture of such isomers.
Known processes for the purification of lactides from the prior art have the disadvantage of being complex, use additional solvents or are inefficient in the sense that their purification potential is limited; typically multiple processes must be executed in series to remove all the impurities. They also generate separate waste streams or use solvents that must be treated and or recovered separately and as a result are prohibitively expensive in terms of process investment and operating cost.
Chinese patent CN 1,757,643 (2006) describes the use of ethyl acetate as solvent for the crystallization of lactides, with the aim to improve the quality of the crystallized product and improve re-crystallization efficiency.
Chinese patent CN 1,757,644 (2006) describes the use of ethanol as solvent for the crystallization of lactides, with the aim to improve the quality of the crystallized product and improve re-crystallization efficiency.
All such methods using a solvent are disadvantageous since they considerably add to the complexity of the process: solvents must be purchased, stored and recovered. Their use must not harm the product in any way and it must not contaminate the environment. Depending on the solvent, their use can also require extensive explosion protection requirements, which additionally adds to the process investment.
U.S. Pat. No. 5,264,592 (1995) describes the use of a melt crystallization process for purification of lactides where the crystals are formed on an interior surface of a crystallizer. With this method the content of the major isomer in the crystallized product is only gradually increased and therefore the frequent repetition of the proposed method is typically compulsory for achieving the required Lactide purities
U.S. Pat. No. 6,310,218 B1 (1996) describes the similar melt crystallization process for purification of lactides which—according to the inventors—can only gradually increase the content of the major isomer in the crystallized product and therefore typically requires the frequent repetition of the proposed method to achieve the desired purities.
Discontinuous or semi-continuous static and dynamic layer crystallization processes as described in the above two patents are known from the prior art for other chemical applications: e.g. acrylic acid, DMT, para- and meta-Xylene among others. Such layer melt crystallization processes are characterized by relatively high crystal growth rates between 10−5 m/s and 10−6 m/s and result in an impure crystal product. The crystal lattice typically would still remain pure, but the crystals are grown on a cooled surface in a dendrite like structure and mother liquor containing all the impurities gets entrapped into the multifaceted structure. It is known that such dynamic impurity inclusion effects become more pronounced with increasing viscosity. The separation of a single layer melt crystallization process can be enhanced by additional purification methods like sweating and washing: these methods offer an increase in purification efficiency at the expense of the product yield. If such a melt crystallization process is performed in an optimized way by a person skilled in the art, the solid phase can typically be 5 to 10 times purer than the liquid melt, in case additional sweating and/or washing is applied this purification ratio can be as high as 20. In the various examples of U.S. Pat. Nos. 5,264,592 and 6,310,218 B1 above, purification ratios between 2 and 20 are disclosed. Apart from the obvious economical disadvantage of the required process repetitions, such replications lead to a lower process yield and to a higher energy consumption. The lower process yield is due to the fact that only a certain portion can be crystallized out of a certain amount of feed. If, for example 50% of the initial mass is crystallized on the heat exchanger surface of the crystallizer and such process is repeated twice, the final product is only 25% of the initial amount of feed. The various intermediate fraction can also be subjected to the method of the above two inventions thereby increasing the yield again but requiring an even more complex process structure with various purity and various recovery stages. The frequent repetition of multiple batch wise process steps with intermediate melting and re-crystallization consumes much more energy than a single stage process. Due to the much longer processing time along with intermediate periods at higher temperatures, the batchwise layer crystallization process is also subject to ring-opening reactions of the lactic by the presence of impurities and thus characterized by increased lactide losses.